It is well known that photographic sensitivity can be heightened by increasing the grain size of silver halide crystals. Increasing the silver halide grain size is often effected by using a so-called silver halide solvent which accelerates growth of silver halide crystal grains during precipitation of silver halides or the subsequent physical ripening. In cases where an emulsion of tabular silver halide grains is used as in the present invention, the silver halide solvent plays a very important role in not only controlling the mean grain size or grain size distribution but also changing the ratio of the grain diameter to the grain thickness.
The silver halide solvents which can be used include nitrogen-containing silver halide solvents the nitrogen atom of which coordinates with a silver ion to accelerate growth of grains as typically exemplified by ammonia, and sulfur-containing silver halide solvents the sulfur atom of which coordinates with a silver ion to accelerate growth of grains, such as thioether compounds, thione compounds and thiocyanates.
Among these silver halide solvents, the nitrogen-containing compounds, e.g., ammonia, can be deactivated by neutralization with acids to lose its coordination with silver ions. In other words, ammonia is characterized by serving as a silver halide solvent for accelerating grain growth only when needed and losing its effect on grain growth upon being neutralized with acids and, therefore, is easy to use. After silver halide crystals are formed in the presence of ammonia, if the ammonia is neutralized with acids, it neither induces unnecessary physical ripening to cause changes of crystals during the subsequent chemical ripening with a chemical sensitizer nor influences the chemical ripening itself. Further, it does not hinder various compounds added until coating, e.g., sensitizing dyes, antifoggants, stabilizers, etc., from adsorption onto silver halide crystals.
However, use of ammonia involves problems such that application is seriously restricted to a high pH condition and also fog is apt to increase. In addition, application of ammonia as a silver halide solvent to tabular grains having a diameter at least 3 times, particularly at least 5 times, the thickness fails to produce grains that can fully manifest their inherent characteristics, such as a high covering power and excellent color sensitizing property. For example, Japanese Patent Application (OPI) No. 108526/83 (corresponding to U.S. Pat. No. 4,435,501 and British Pat. No. 2,111,231) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and Japanese Patent Application (OPI) No. 113928/83 (corresponding to U.S. Pat. No. 4,434,226 and British Pat. No. 2,109,576) describe ammonia as being an unfavorable physical ripening agent in a silver iodobromide emulsion containing tabular grains having a large diameter/thickness ratio (these patents refer to this ratio as the "aspect ratio"). Accordingly, ammonia in the state of the art is undesirable as a silver halide solvent in the preparation of tabular silver halide emulsions.
On the other hand, the sulfur-containing silver halide solvents, such as thioether compounds, thione compounds, thiocyanates, etc., are preferred for the preparation of tabular silver halide grains. However, it has hitherto been impossible to deactivate these sulfur-containing solvents to cause them to lose their effect except for removal by washing with water. Washing for ceasing the grain growth effect entails a great increase in both cost and time for the production of silver halide emulsions and is, thereofre, unsuitable for practical operation. Moreover, these sulfur-containing silver halide solvents cannot be completely removed even by washing with water and some portion remains in the emulsion because of the strong affinity of the sulfur-containing solvents for silver halide grains compared with ammonia. The silver halide solvents remaining in the emulsion produce various adverse effects during chemical ripening. For example: fog is increased; physical ripening proceeds simultaneously with chemical ripening to cause the disappearance of sensitivity specks on the surface of the grains; chemical ripening is hard to stop by cooling or with adsorbing additives; and the like. The residual silver halide solvents also promote deterioration of photographic performance properties during preservation or hinder various additives, such as sensitizing dyes, from adsorption.
Nevertheless, sulfur-containing silver halide solvents facilitate mono-dispersion or tabular silver halide grains having a large diameter/thickness ratio as compared with ammonia as mentioned above and, above all, realize preparation of tabular silver halide emulsions having high photographic sensitivity. In addition, the sulfur-containing silver halide solvents have various advantages in that uniform distribution of iodine in a silver iodobromide emulsion is easily accomplished; growth of grains is accelerated even at a low pH level; silver halide grains relatively insensitive to pressure applied on films can be produced; and so on.
For all these reasons, it has been desired to develop a method capable of reducing or eliminating the grain growth effect of the sulfur-containing silver halide solvents whenever required as is achieved by using acids against ammonia.